An apparatus for optically combining at least three light beams can be applied for example in a telescope arrangement that is arranged to make light from a main direction interfere destructively at a null output. This can be used to observe light received from a different direction when a dominant light source is present in said main direction. In astronomy, for example, the light from a planet could be observed near a star, by making the light from the star interfere destructively.
To ensure destructive interference for light from the main direction it is desirable to use a control loop for adjusting the optical path lengths for the different beams relative to one another. Controllable optical path lengths are provided in front of the beam combiners where the beams are combined. The control loop ensures that before they enter the beam combiners the beams have relative phases for light from the main direction so that this light will interfere destructively in the null path.
In one possible set-up of the control loop, the control loop contains beam splitters to split off fringe tracking parts of the beams before the remainder of the beams is combined. The split off fringe tracking beams are combined so that they interfere at least partly constructively. The measured intensity of the constructively interfering fringe tracking beams depends on the phase relation between the beams for light from the main direction. This measured intensity is used to determine whether the beams have the proper phase relation for destructive interference in the null path and, if not, how to change the optical path length to reach the proper phase relationship.
However, this possible set-up has the disadvantage that it requires close control over the properties of the part of the beam paths beyond the point where the fringe tracking beams are split off, as far as these properties affect the light. This part of the optical system must be extremely stable, or it must be kept stable at the expense of a complex metrology system. This makes such a set-up heavy, expensive and complex.